The invention relates to a method for packaging a circuit containing microwave components or microwave components and digital components, the components being placed on a printed circuit.
For packaging a circuit of the above type use is made in the art of a hermetic metal housing, with at least the microwave components being placed unencapsulated in an inert gas. It is actually an ideal package, but it is also an expensive one, because each connection must be realized via a glass bead in the housing wall. Especially if there are many connections, for example carrying digital control signals, the cost of the packaging becomes prohibitive and thus impractical.
A further drawback of the known packaging method is that the package is considerably heavier and bulkier than the circuit proper. This drawback is felt in particular with applications for aircraft.
Of exposed circuits with the exception of microwave components it is known that they can be protected against moisture and corrosion by providing them with a top layer of lacquer, for example a polymer lacquer. For the protection of microwave components this is not possible, because the top layer introduces dielectric losses, causing the circuit to function suboptimally or not at all. Besides, a circuit packaged in this way is very susceptible to moisture, as any moisture in the vicinity of the circuit occasions losses of such magnitude that the circuit no longer operates.
The method according to the invention meets these objections, and is characterized in that at least the microwave components are covered with a layer of syntactic foam comprising a matrix of epoxy resin or cyanate ester, filled with microballoons of glass or a ceramic material, the circuit subsequently being covered with a moisture-proof top layer. The syntactic foam layer reduces the dielectric losses as only very little dielectric is present in the vicinity of the microwave components, at the same time enlarging the distance between the microwave components and any moisture present in the top layer.
An advantageous realization of the inventive method is characterized in that in the syntactic foam 50-80% of the volume is occupied by microballoons. Syntactic foam of this composition is sufficiently robust, causes little dielectric losses and is exceptionally light. Compared with a circuit packaged hermetically in a known manner, the volume and weight are at least halved.
A further advantageous realization of the inventive method is characterized in that during curing of the matrix the circuit is held in a position such that an upward pressure due to the gravitational force or a centrifugal force will guide the microballoons to the microwave components. Thus, the relatively large concentration of microballoons in the matrix near the microwave components will reduce the dielectric losses further still, while the mechanical properties of the syntactic foam remain virtually the same.
A preferred realization of the inventive method, having as an additional advantage that on the one hand electromagnetic radiation cannot penetrate to the microwave components and on the other that the microwave components are prevented from emitting electromagnetic radiation, is characterized in that after application of the top layer, at least at the spots where the microwave components are situated the top layer is provided with a metallization.
A further advantageous realization of the inventive method is characterized in that the metallization is applied in the form of a conducting paint or by depositing metal particles through sputtering. An alternative advantageous realization is characterized in that the metallization is realized through fitting a metal plate over the syntactic foam layer.
The invention also relates to a package for a printed circuit comprising microwave components or microwave components and digital components. It is characterized in that the package for the microwave components includes a layer of syntactic foam, and that the printed circuit, including the syntactic foam layer is provided with a moisture-proof top layer.